<!DOCTYPE html>
<html lang="zh-cn">

<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Document</title>
</head>

<body></body>

</html>
<script>
  console.log(typeof undefined === "undefined"); //true
  console.log(typeof true === "boolean"); //true
  console.log(typeof 42 === "number"); //true
  console.log(typeof "42" === "string") //true
  console.log(typeof { life: 42 } === "object"); //true
  console.log(typeof Symbol() === "symbol"); //true
  console.log(typeof null === "object"); //true
</script>